Imagine you're at the beach and you see your friend struggling in the water. You want to help him quickly. But do you go straight into the water or run around the beach first? It turns out the best path is somewhere in between. But how does light know which path to take when it goes from one medium to another? It's a strange question. Light doesn't just change direction when it hits an interface. It actually explores all possible paths at the same time. And the path we see is an illusion.
